Voltage Drop Calculations- Part One

1- Introduction:

What is the
Voltage Drop?

Wires carrying current always have inherent resistance, or impedance,
to current flow. Voltage drop is defined as the amount of voltage loss that
occurs through all or part of a circuit due to impedance. (see below image)

Why to calculate
the Voltage Drop?

Voltage drop becomes important for the following reasons:

a- Insure more System efficiency: An applied voltage of 10 percent
below rating can result in a decrease in efficiency more than 10 percent

For example, fluorescent light output would be reduced by 15 percent,
and incandescent light output would be reduced by 30 percent.

Induction motors would run hotter and produce less torque. With an
applied voltage of 10 percent below rating, the running current would
increase 11 percent, and the operating temperature would increase 12 percent.
At the same time, torque would be reduced 19 percent.

b- Enhance Troubleshooting: If you follow the voltage drop
recommendations, you don’t have to guess whether your field measurements
indicate a problem or if the voltage is low due to not accommodating voltage
drop in the design.

c- Enhance Load protection: Under-voltage for inductive loads can
cause overheating, inefficiency, and shorter life span. The voltage-drop
requirements are needed to ensure the operation of over-current devices to
protect conductors and equipment supplied by these systems.

a- The impedance of the conductor: The voltage drop of a circuit is in
direct proportion to the impedance of the conductor.

If you increase the length of a conductor, you increase its
resistance—and you thus increase its voltage drop. Thus long runs often
produce voltage drops that exceed NEC recommendations.

b- The magnitude of the load current: The voltage drop of a circuit is
in direct proportion to The magnitude of the load current. If you increase
the load current, you increase the conductor voltage drop.

c- Conductor Material: Copper is a better conductor than aluminum and
will have less voltage drop than aluminum for a given length and wire size.

d- Conductor Size: Larger wire sizes (diameter) will have less voltage
drop than smaller wire sizes (diameters) of the same length.

e- Conductor Length: Shorter wires will have less voltage drop
than longer wires for the same wire size

f- The load power factor,

g- The type of raceway or cable enclosure,

h- The type of circuit (ac, dc, single phase, 3-phase).

2- NEC voltage drop Recommendations

The NEC recommendations concerning Voltage Drop are as follows:

Section 210.19(A)(1), Informational Note No. 4

Section 215.2(A)(4), Informational Note No. 2,

310.15(A)(1), Informational Note No. 1

And as per NEC 90.5(C), Informational Notes are recommendations, not requirements.

Rule#1

Contrary to common belief, the NEC generally does not require you to
size conductors to accommodate voltage drop. It merely recommends that you
adjust for voltage drop when sizing conductors.

As per NEC Section 210.19(A)(1), Informational Note No. 4 and Section
215.2(A)(4), Informational Note No. 2, Conductors for branch circuits as
defined in Article 100, sized to prevent a voltage drop exceeding 3 percent
at the farthest outlet of power, heating, and lighting loads, or combinations
of such loads, and where the maximum total voltage drop on both feeders and
branch circuits to the farthest outlet does not exceed 5 percent, provide
reasonable efficiency of operation.(see below image)

Rule#3

As
per NEC Section 310.15 Ampacities for Conductors Rated 0–2000 Volts, Ampacities
provided by this section do not take voltage drop into consideration.

The recommended Values for the voltage drop as per NEC code for different applications as in the following table:

Total
circuit voltage drop = sum of the voltage drops of each serial circuit
segment, where the load for each segment is calculated using the load that segment
sees and the demand factors of 551.73(A).

For conductors 1/0 AWG and smaller, the difference in resistance between dc and ac circuits is so little that it can be ignored. In addition, you can ignore the small difference in resistance between stranded and solid wires.

Example#1:

What is the voltage drop of two 12 AWG THHN conductors that supply a 16A, 120V load located 100 ft from the power supply?

The maximum voltage drop allowed is 9.5 V. To determine the size of cable that will be required to meet the voltage drop requirement, determine the value of R that will meet the requirement. 9.5 = R x 100 x 150 / 1000

R = 9.5 x 1000/100 x 150 = 0.633 mV/A/m

Referencing the table provided by the cable manufacturer (Table 1), the cable that has a resistance of 0.633mV/A/m or less is a 70 mm cable with a resistance of 0.55 mV/A/m.